1. Field of the Invention
The present invention relates to semiconductor devices having a memory element and methods of manufacturing semiconductor devices having a memory element.
Note that in the present invention, the term ‘semiconductor device’ refers to a device having a circuit which includes a semiconductor element (a transistor, a diode, or the like).
2. Description of the Related Art
In modern society, where many electronic devices are used, various data are generated and used, and memory elements (hereinafter also referred to as ‘memories’) are required to store the data. Various memories manufactured and used each have advantages and disadvantages, and are used appropriately depending on the type of data which is stored and used.
Memories can be broadly divided into two types; that is, volatile memories and nonvolatile memories. A volatile memory is a memory which loses stored data when the power is turned off, and a nonvolatile memory is a memory which retains stored data even when the power is burned off. Examples of volatile memories are dynamic random-access memories (DRAMs) and static random-access memories (SRAMs). The application of volatile memories is significantly limited because stored data is lost when the power is turned off; however, because the amount of time required for access is short, they are used for cache memories of computers and the like. Since a DRAM has small memory cells, a large-capacity DRAM can be easily produced. However, it is controlled in a complex manner and consumes a lot of power. An SRAM memory cell includes a CMOS transistor and is easily manufactured and controlled; however, since six transistors are necessary for one memory cell, it is difficult to obtain a large-capacity SRAM.
Nonvolatile memories, which retain their stored data even after the power is turned off, can be broadly divided into three types; that is, rewritable memories, write-once memories, and mask ROMs (read-only memories). Stored data can be rewritten many times, up to a certain limit, in rewritable memories. A user of a write-once memory can write data to the write-once memory only once. For a mask ROM, data content is determined at the time of manufacturing the memory, and cannot be rewritten.
Examples of rewritable nonvolatile memories are EPROMs, flash memories, ferroelectric memories, and the like. EPROMs allow easy writing of data and unit cost per bit is relatively low; however, they require a program device and an eraser dedicated to writing and erasing. In flash memories and ferroelectric memories, data can be rewritten on a substrate used. Flash memories and ferroelectric memories have short access time and low power consumption.
An example of a structure of a flash memory is a structure in which a tunnel insulating film, a floating gate, a gate insulating film, and a control gate are formed over an active layer.
Further, for such a flash memory, in order to increase response speed, scaling down design rules or forming silicide in a source region and a drain region of the active layer so that resistance of the source and drain regions themselves and contact resistance with a wiring are reduced can be considered. In order to form the silicide, a metal film formed of nickel (Ni), tungsten (W), titanium (Ti), cobalt (Co), or the like is used (refer to Patent Document 1: Japanese Published Patent Application No. 2006-13481 and Patent Document 2: Japanese Published Patent Application No. 2006-32917).